SQL MAX aggregate function not bringing the latest date - sql

Question

Purpose: I am trying to find the max date of when the teachers made a purchase and type.
Orders table
ID
Ordertype
Status
TeacherID
PurchaseDate
SchoolID
TeacherassistantID
1
Pencils
Completed
1
1/1/2021
1
1
2
Paper
Completed
1
3/5/2021
1
1
3
Notebooks
Completed
1
4/1/2021
1
1
4
Erasers
Completed
2
2/1/2021
2
2
Teachers table
TeacherID
Teachername
1
Mary Smith
2
Jason Crane
School table
ID
schoolname
1
ABC school
2
PS1
3
PS2
Here is my attempted code:
SELECT o.ordertype, o.status, t.Teachername, s.schoolname
,MAX(o.Purchasedate) OVER (PARTITION by t.ID) last_purchase
FROM orders o
INNER JOIN teachers t ON t.ID=o.TeacherID
INNER JOIN schools s ON s.ID=o.schoolID
WHERE o.status in ('Completed','In-progress')
AND o.ordertype not like 'notebook'
It should look like this:
Ordertype
Status
teachername
last_purchase
schoolname
Paper
Completed
Mary Smith
3/5/2021
ABC School
Erasers
Completed
PS1
2/1/2021
ABC school
It is bringing multiple rows instead of just the latest purchase date and its associated rows. I think i need a subquery.

Answer 1

Aggregation functions are not appropriate for what you are trying to do. Their purpose is to summarize values in multiple rows, not to choose a particular row.
Just a window function does not filter any rows.
You want to use window functions with filtering:
SELECT ordertype, status, Teachername, schoolname, Purchasedate
FROM (SELECT o.ordertype, o.status, t.Teachername, s.schoolname,
o.Purchasedate,
ROW_NUMBER() OVER (PARTITION by t.ID ORDER BY o.PurchaseDate DESC) as seqnum
FROM orders o JOIN
teachers t
ON t.ID = o.TeacherID
schools s
ON s.ID = o.schoolID
WHERE o.status in ('Completed', 'In-progress') AND
o.ordertype not like 'notebook'
) o
WHERE seqnum = 1;

Answer 2

You can use it in different way. it's better to use Group By for grouping the other columns and after that use Order by for reorder all records just like bellow.
SELECT top 1 o.ordertype, o.status, t.Teachername, s.schoolname
,o.Purchasedate
FROM orders o
INNER JOIN teachers t ON t.ID=o.TeacherID
INNER JOIN schools s ON s.ID=o.schoolID
having o.status in ('Completed','In-progress')
AND o.ordertype not like 'notebook'
group by o.ordertype, o.status, t.Teachername, s.schoolname
order by o.Purchasedate Desc